Navigation

Behaviour of silver nanoparticles in wastewater

 

Silver nanoparticles used in various commercially available products can get into the wastewater and thus into water treatment plants when these products are used. Ralf Kaegi and his team examined the persistence and important transformation processes of silver nanoparticles in sewage plants.

Background (ongoing research project)

Due to their antimicrobial effect, silver nanoparticles are increasingly used in consumer products such as textiles and cosmetics, but also for medical applications. When these products are used, silver nanoparticles are released and thus enter, for example, waste water systems and then water treatment plants. Depending on how the particles behave in the water treatment plant, they can either pass through the process and get into surface water or accumulate in sewage sludge. With his research team, Ralf Kaegi investigated the behaviour and conversion processes of silver nanoparticles in water treatment plants and in urban surface water. The antimicrobial effect of the silver may impair important biological processes of water treatment plants. However, it is known that silver nanoparticles lose this toxic effect as soon as they bind with sulphides. As a result, the research group investigated whether silver nanoparticles combine with sulphides that occur naturally in wastewater. As wastewater is often treated with ozone to eliminate residual pollutants, the team also investigated whether this ozonation affects the sulphidation of silver nanoparticles.

Results

Silver nanoparticles were added to a sewer pipe. Water samples were then taken from various collection points along the 5 km sewage system and analysed for silver nanoparticle content. The experiments showed that silver nanoparticles entering the sewage system via wastewater are transported very efficiently to the sewage treatment plant and only an insignificant part of the particles remain attached to the walls of the sewer pipes. In the wastewater, the silver nanoparticles are very quickly mixed with other particles and 95% end up in sewage sludge, which is burned in Switzerland. Using X-ray absorption spectrometry, the group was able to demonstrate that the silver nanoparticles bind very rapidly with sulphides, both on the way through the sewage system, in the sewage treatment plant, as well as in surface water. When wastewater is treated with ozone, silver sulphide is oxidized and silver ions are released. However, these silver ions break down rapidly into silver chlorides, which in turn reduces the concentration of free silver ions in the treated wastewater. In summary, the project presents several mechanisms by which silver nanoparticles are very efficiently eliminated from the wastewater or end up as compounds with a greatly reduced toxicity. A toxic effect of the ozone treated wastewater was only observed in experiments where unrealistically high concentrations of silver nanoparticles were used. Further experiments with different types of nanoparticles and with nanoparticles of different sizes and coatings suggest that purification plants represent a very efficient barrier for various types of (metallic) nanomaterials.

Relevance

The project provides an improved understanding of the physical and chemical processes that determine the behaviour and retention of silver nanoparticles in water treatment plants and thus provides an important basis for the development and regulation of these plants, e.g. not to use sewage sludge as a fertilizer or the ozone treatment of sewage water.

Original title

Behavior of silver nanoparticles in a wastewater treatment plant

Project leader(s)

  • Dr. Ralf Kägi

Co-applicants

  • Prof. Eberhard Morgenroth
  • Dr. Andreas Voegelin

 

 

Further information on this content

 Contact

Dr. Ralf Kägi Eawag Überlandstrasse 133
Postfach 611
8600 Dübendorf +41 44 823 50 45 ralf.kaegi@eawag.ch